Preserve letterbox while dragResizing

Fixes an issue where the letterbox would disappear while entering
and leaving dragResizing, because this clears the hasDrawnState.

To fix this, also treat preserved surfaces as ready to show for
letterboxing purposes.

Change-Id: I4d6e1b1a27d99542370fde8924f126eca1ea80d5
Fixes: 76008154
Test: Enable double cutout, enter split screen, go home, touch slider verify the letterbox does not flicker.